In recording/reproduction apparatuses for recording original digital information on, or reproducing such information from, a portable recording medium, there can be a variance in the shape of marks formed on the medium among individual apparatuses or recording mediums even with an identical shape of recording pulse. This results in significant difference in the quality of the signal reproduced. In order to avoid reduction in the reliability due to the variance, a correction operation is performed when, for example, the recording medium is mounted. A correction operation is a control operation for optimizing the setting of characteristics of the reproduction system, the shape of the recording pulse, or the like, in order to guarantee the reliability of user data.
A general information reproduction apparatus includes a PLL circuit for extracting clock information included in a reproduced signal and identifying the original digital information based on the clock information extracted.
FIG. 33 shows a conventional optical disc drive. Light reflected by an optical disc 17 is converted into a reproduced signal by an optical head 18. The reproduced signal is shape-rectified by a waveform equalizer 19. The resultant reproduced signal is binarized by a comparator 20. Usually, the threshold of the comparator 20 is feedback-controlled such that an integration result of binary signal outputs is 0. A phase comparator 21 obtains phase errors between the binary signal outputs and the reproduction clocks. The phase errors are averaged by an LPF 22, and a control voltage of a VCO 23 is determined based on the processing result. The phase comparator 21 is feedback-controlled such that the phase errors output by the phase comparator 21 are always 0. In recording mediums on which information is thermally recorded, the shape of the marks formed thereon vary in accordance with the thermal interference of the mediums and recording patterns before and after the mark which is to be recorded. Therefore, a recording parameter which is optimum for the recording of each pattern needs to be set.
The above-described error detection output is an index for evaluating the recording parameter. The recording parameter is set such that the error detection output is as small as possible. Specifically, a recording compensation circuit 27 generates a pulse having a prescribed pattern based on a recording pattern which is output from a pattern generation circuit 26 using an initially set recording parameter. A laser driving circuit 28 records information on the optical disc. While information is being reproduced from a track having the prescribed pattern recorded thereon, an error detection circuit 24 integrates absolute values of phase errors between an output from the comparator 20 and an output from the VCO 23, and thus obtains a detection signal. The detection signal is correlated with jitter between a reproduction clock and a binarized pulse edge. Recording and reproduction are repeatedly performed with different recording parameters. The recording parameter used when the detection value is minimum is determined as an optimum recording parameter.
FIG. 34 shows a specific operation of the error detection circuit 24. Here, a recording pattern having a repetition of 6T, 4T, 6T and 8T is used. The mark termination edge corresponding to a pattern of a combination of 4T marks and 6T spaces is optimized. It is assumed that a mark start edge corresponding to a pattern of a combination of 6T spaces and 8T marks, and a mark termination edge corresponding to a pattern of a combination of 8T marks and 6T spaces, are recorded with an optimum recording parameter.
When given an NRZI signal having a period shown in FIG. 34(a), the recording compensation circuit 27 generates a laser driving waveform pulse shown in FIG. 34(b). Tsfp is a parameter for setting a mark start position, and Telp is a parameter for setting a mark termination position. The laser driving circuit 28 modulates light emitting power in accordance with the pattern shown in FIG. 34(b). An amorphous area is physically formed on the track as shown in FIG. 34(c) by laser light. When Telp is varied as Telp1, Telp2 and Telp3, the shape of the mark formed is changed as shown in FIG. 34(c). Information reproduction from the track having such marks will be discussed.
When the recording parameter at the end of the 4T mark is Telp2, which is the optimum value, a reproduced signal shown with a solid line in FIG. 34(d) is obtained. The threshold value is defined such that the integration value of the outputs from the comparator is 0. A phase difference between the output from the comparator and the reproduction clock is detected, and a reproduction clock (FIG. 34(e)) is generated such that the integration value of the phase errors is 0.
In the case where the recording parameter at the end of a 4T mark is made Telp1, which is smaller than the optimum value, a reproduced signal shown in FIG. 34(f) with the solid line is obtained. Since the termination edge of the 4T mark changes in a time axis direction, the threshold value Tv of the comparator is greater than in the reproduced signal shown in FIG. 34(d), as indicated by the one-dot chain line in FIG. 34(f). Because of the change in the output from the comparator, the phase of the reproduction clock is advanced as compared to the reproduction clock shown in FIG. 34(e) such that the integration value of the phase errors is 0. As a result, a reproduction clock shown in FIG. 34(g) is generated.
By contrast, in the case where the recording parameter at the end of a 4T mark is made Telp3, which is greater than the optimum value, a reproduced signal shown in FIG. 34(h) with the solid line is obtained. Since the termination edge of the 4T mark changes in a time axis direction, the threshold value Tv of the comparator is smaller than in the reproduced signal shown in FIG. 34(d), as indicated by the one-dot chain line in FIG. 34(h). Because of the change in the output from the comparator, the phase of the reproduction clock is behind as compared to the reproduction clock shown in FIG. 34(e) such that the integration value of the phase errors is 0. As a result, a reproduction clock shown in FIG. 34(i) is generated.
Measurement results of the time difference between the mark termination edge (rising edge of a reproduced signal) and the reproduction clock (so-called data-clock jitter) exhibit distributions shown in FIG. 34(j) through (l). It is assumed here that the 4T mark termination edge and the 8T mark termination edge have a variance such that both of the edges exhibit normal distributions of identical variance values.
In the case of the reproduced signal shown in FIG. 34(d), and the reproduction clock shown in FIG. 34(e), the time difference distribution between the output from the comparator and the reproduction clock at the rising edge (mark termination edge) is as shown in FIG. 34(k). The average value of the distributed values at the 4T mark termination edge, and the average value of the distributed values at the 8T mark termination edge, are each 0.
In the case where the parameter of the end of the 4T mark is Telp1 (smaller than the optimum value Telp2), neither the average value of the distributed values at the 4T mark termination edge, nor the average value of the distributed values at the 8T mark termination edge, is 0, but both are away from 0 by the same distance, as shown in FIG. 34(j). Therefore, the total variance at the rising edge is greater than the case in FIG. 34(k). Similarly, in the case where the parameter of the end of the 4T mark is Telp3 (greater than the optimum value Telp2), neither the average value of the distributed values at the 4T mark termination edge, nor the average value of the distributed values at the 8T mark termination edge, is 0, but both are away from 0 by the same distance, as shown in FIG. 34(l). In FIG. 34(j) and (l), the distribution of the 4T mark termination edge and the distribution of the 8T mark termination edge are inverted. In this case also, the total variance at the rising edge is greater than the case in FIG. 34(k).
In the case where the accumulation result of absolute values of phase errors is the error detection output, the error detection value changes as shown in FIG. 34(m) in accordance with the change in the recording parameter Telp. Accordingly, the recording parameter is varied, and the parameter when the output from the error detection circuit 24 is minimum is determined as an optimum parameter.
In the above example, the parameter Telp at the 4T mark termination edge is optimized. For the other parameters, test recordings using a respective specific parameter are performed and the optimum parameters are obtained based on the error detection output.
FIG. 35 is a flowchart illustrating an operation for obtaining all the recording parameters in accordance with the above-described procedure. Areas of a medium on which test recordings are to be performed are accessed (S161), and the test recordings are performed while the recording parameter at the mark start edge or the mark termination edge is changed prescribed area by prescribed area (for example, sector by sector)(S163). Information is reproduced from the test recording areas, and error detection outputs are obtained area by area by which the parameter is changed (S164). The parameter at which the error detection output is minimum is determined as an optimum parameter (S165). This operation is repeated until all the optimum parameters are obtained (S162) in order to obtain the next parameter (see Patent Document No. 1 and Patent Document No. 2).
Patent Document No. 1: Japanese Laid-Open Patent Publication No. 2000-200418
Patent Document No. 2: Japanese Laid-Open Patent Publication No. 2001-109597